Method and apparatus for adjusting interelectrode spacing in a cathode-ray tube



July 22, 1969 s. M. SCHRIJNEMAKERS 3,456,311

METHOD AND APPARATUS FOR ADJUSTING INTERELECTRODE SPACING IN A CATHODE-RAY TUBE Filed Nov. 14, 1966 2 Sheets-Sheet 1 AGET y 1969 G. s. M. SCHRIJNEMAKEERS METHOD AND APPARATUS FOR ADJUSTING INTERELECTRODE SPACING IN A CATHODE'RAY TUBE Filed Nov. 14, 1966 2 Sheets-Sheet 2 INVFNTOR.

GERARDUS 5. M. SCHRHNENAKERS 25% a AGENT United States Patent ()1 ficc 3,456,31 1 Patented July 22, 1969 US. Cl. 29-2518 5 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for adjusting the spacing between the cathode and an adjacent electrode in a cathoderay tube by removal of a portion of the cathode and measuring changes in capacitance value between the cathode and the adjacent electrode.

The invention relates to a method and an arrangement for manufacturing an oxide-coated cathode for use in a cathode-ray tube in which the distance from the emissive surface to an adjacent electrode is adjusted to a desired low value by capacitance measurement. The invention relates more particularly to a method and an arrangement in which an excessively thick oxide layer is reduced mechanically to the desired thickness.

The thickness of an emissive layer of a cathode can be reduced by mechanical operations such as pressing, scraping, planing, grinding or milling. Furthermore, the distance from the emissive layer of an oxide-coated cathode to an adjacent or control electrode can be adjusted by controlling the capacitance between the cathode and this adjacent electrode or an electrode disposed behind it, i.e., first anode. The mechanism for modifying the distance from the cathode surface to the adjacent electrode can be controlled by the capacitance between the cathode and an adjacent electrode. When this capacitance has reached a predetermined value, the arrangement is stopped.

If the distance between the cathode and the adjacent electrode is adjusted by adjusting the capacitance between the cathode and the subsequent electrode to a given value, this distance between cathode and control electrode depends upon the remaining tolerances of the mechanical dimensions of the system such as the diameter of the opening and the thickness of the material of the control electrode and the first anode and their relative distances.

This capacitative adjustment ensures that the influence of these tolerances on the value of the control voltage required for reducing the electron current completely to zero is strongly decreased.

However, the capacitance between the cathode and the first anode is also determined by the ratio between the thickness and the dielectric constant of the emissive layer. Since the dielectric constant of the layer depends upon the density of this layer and hence also upon the shrinkage occurring during drying of the layer after the application of the oxides from a suspension, this dielectric constant is not always the same. Consequently, the thickness of the carbonate layer is not always constant and variations are obtained in the distance between the surface of the layer and the adjacent electrode, while the fluctuation of the cut-off voltage also increases.

In accordance with the invention, the thickness of an originally excessively thick emissive layer is reduced in dependence upon the dielectric constant of the layer so that the quotient of layer thickness and dielectric constant ultimately exhibits a predetermined value which avoids the aforesaid disadvantages.

A dense layer having a high dielectric constant must have a proportionally greater thickness. Conversely, the surface of the emissive layer must always lie at the same distance from the adjacent electrode after the capacitatlve adjustment of the cathode with respect to the subsequent electrodes of an electron gun, since in the case of a layer of greater density, the preadjusted capacitative value is already reached when the cathode support still is at a greater distance from the adjacent electrode. Consequently, when the layer of greater density has a greater thickness, its surface lies at the same distance from this electrode as the surface of a layer of smaller density and hence, of smaller thickness despite the greater distance between the support and the adjacent electrode. The cathode support is located closer to the adjacent electrode when the predetermined capacitance value is reached. The thickness of the originally excessively thick layer may be reduced by pressing; in this case, however, the density of the carbonate layer does not increase uniformly but the upper layer is compressed to a greater extent than the parts of the layer lying closer to the cathode support.

In another method the layer is scraped or planed in known manner by means of a sharp knife. In this embodiment of the invention, the holder of the knife also includes an electrode whose capacitance with respect to the cathode support and the emissive layer is measured. After each scraping movement, the increase in capacitance is measured and the scraping movement may be repeated depending upon whether the measured capacitance still deviates from the adjusted measuring value. Moreover, in this method, the roughness of the emissive layer is completely eliminated.

The invention will now be described more fully with reference to the drawing, in which:

FIG. 1 shows diagrammatically an arrangement for gradually planing an excessively thick emissive layer to the desired thickness, and

FIG. 2 shows diagrammatically a suitable bridge circuit for measuring the capacitance between the cathode and an adjacent electrode. Referring now to FIG. 1, a metal support 1 carries an emissive layer 2 of a hood-shaped cathode for use in a cathode-ray tube. Cathode support 1 is held, for example, by vacuum on a supporting member 3 which is centered in cathode tube 4 by means of a centering member 5. Cathode tube 4 is welded to metal support 1. Emissive layer 2 is applied so that it has an excessively great thickness.

Above cathode support 1 a holder '6 provided with a knife 7 is arranged which may act as a plane. Furthermore, holder 6 is provided with a central electrode 8 which is secured in holder 6 by means of insulating material 9. The position of knife 7 with respect to electrode 8 and holder 6 is adjustable so that emissive layer 2 and electrode 8 may be separated by a layer of air. Holder 6 and supporting member 3 with cathode 1 can be relatively moved to and fro and can be caused to move more or less closely towards each other by a fine adjustment. An alternating voltage supplied by a bridge circuit is applied to electrode 8 and cathode 1 and the emissive layer is gradually planed until the capacitance between electrode 8 and cathode 1 reaches a pre-determined measured value. The movement of cathode 1 with respect to holder 6 can be stopped automatically or manually when the measured value is reached. In the case of automatic control, a servomechanism is connected between the bridge circuit and the driving means.

thelevel difference between the knife and -electrode--- 6 with electrode 8 must be varied in order to obtain the desired condition after a second planing movement.

The second planing operation can be carried out in a few steps if this should be required with a view to the structure of the carbonate layer.

Y Electrode 8 preferably has an operative diameter which corresponds with the opening in the first electrode of the electron gun into which the cathode must be incorporated and the position of the upper surface of the emissive layer .with respect to this electrode is adjusted. Consequently,

the thickness of the emissive layer is reduced in dependence upon the density of this layer and hence upon the dielectric constant thereof in a manner such that a layer of greater density obtains a greater thickness. With a higher value of the dielectric constant, the capacitance caused by this layer increases more rapidly so that the pre-deterrnined capacitance value between the cathode and the electrode 8 is already reached when the support of the cathode 1 still lies at a greater distance from the electrode 8. Thus, when the cathode is capacitatively adjusted in the electron gun, the position of the support varies in dependence upon the density of the emissive layer but when the pre-determined capacitance value is reached, the distance between the surface of this layer and the adjacent electrode will invariably be the same. In the arrangement of FIG. 1, the material of the emissive layer 2 removed by scraping or planing is drawn away through a pipe 10.

In the bridge circuit shown in FIG. 2, the alternating current of the source of supply 10 is applied to the branch connected to ground and including the impedances 12 and 13 and to the branch including a capacitor 14 and the planing device. If the impedance between the points 1 and 8 exceeds by many times the impedance 12, the voltage between the holder 6 and the electrode 8 is so low that the system 1-6-8 may be considered as a capacitative screening ring, which, as stated above, is a condition. The bridge is first adjusted by meansof a device 11 having a given measuring capacity and subsequently the capacitance between the electrode 8 and the cathode 1. The difference voltage supplied by the bridge are visualized on a measuring instrument 17 by means of a transformer and a rectifier and amplifier 16, or are applied to a servoapparatus.

--While the:invention has been described in connection with particular embodiments and applications thereof, other modifications such as cathodes of different shape will be obvious to those skilled in this art without departing fromthe spirit and scope of the invention which is defined in the "appended claims.

What is claimed is:

1. A method of manufacturing a cathode having an emissive coating of electron emittingoxides for use in a cathode-ray tube comprising the steps of mechanically reducing an excessively thick oxide layer, while comparing the quotient of layer thickness of dielectric constant with that of 'a layer of given thickness and dielectric constant to thereby control the thickness of the layer being reduced to a desired value. V

2. A method as claimed in claim 1, in which the capacitance between a measuring electrode and the cathode with the emissive layer is measured to determine the thickness of the emissive oxide layer.

3. A method as claimed in claim 2, in which portions of the layer are periodically removed, the capacitance between the cathode and the measuring electrode being measured after each removal, and the removal of portions of the layer being halted when a predetermined value of the capacitance is reached.

4. A method as claimed in claim 3, in which the portions of the layer are removed by planing.

5. A method as claimed in claim 1, in which the layer is pressed to reduce the thickness thereof to a value at which the quotient of the thickness and the dielectric constant of the layer assumes a predetermined value.

References Cited UNITED STATES PATENTS I 2,321,413 6/1943 Palmer 29-30 2,700,000 1/ 1955' Levi et al 29-25.18 2,986,799 6/1961 Levi etal 29-25.18 3,045,320 7/ 1962 Apelbaum et al 29-25.18 3,150,021 9/1964 Sato 29-574 XR 3,261,082 7/ 1966 Maissel et a1 29-620 3,284,878 11/1966 Best 29-620 3,299,332 1/1967 Saburi 317-238 3,330,696 7/1967 Ullery et al. 29-620 JOHN F. CAMPBELL, Primary Examiner R. B. LAZARUS, Assistant Examiner U.S. Cl. X.R. 29-574, 620 

